L1/L2 GPS receiver

ABSTRACT

In a system and method for simultaneously receiving or switching between dual frequency carrier signals in a GPS receiver, the GPS receiver is adapted to utilize different harmonics of a sub-harmonic frequency generator, which may include a lower frequency voltage controlled oscillator (VCO) to detect the L 1  and L 2  GPS carriers. A sub-harmonic mixer may be used to simultaneously down convert the L 1  and L 2  signals to a lower intermediate frequency (IF). A second mixer may be an image reject (IR) mixer used to separate the downconverted L 1  and L 2  signals. This mixer may be configured to simultaneously monitor the L 1  and L 2  signals, or to switch between the L 1  and L 2  signals. High frequency switching is not required of the radio frequency (RF) input or local oscillator signals, and simultaneous L 1  and L 2  reception is enabled without a 3dB image noise degradation. This system and method minimizes the RF components and power dissipation in a dual frequency GPS receiver, while optimizing the functionality and performance.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention:

[0002] The invention relates generally to radio frequency receivers, andmore specifically to multiple band global positioning system (GPS)receivers used for navigation.

[0003] 2. Description of the Related Art:

[0004] GPS satellites transmit data at two radio frequency (RF) systemcarrier frequencies: 1575.42 MHz (L1) and 1227.6 MHz (L2). GPS data fromboth carriers can be used to increase the position accuracy, and toprovide carrier selectivity in case of interference or jamming of one ofthe carriers.

[0005] A GPS receiver designed to receive the L1 and/or L2 carriersrequires a method for receiving both signals simultaneously orefficiently switching between the signals. One solution is to duplicateall receiver parts and functions for the L1 and L2 bands. However, forlow-power portable receivers, it is desirable to integrate the L1 and L2functions as much as possible, to minimize the number of RF functionsand power dissipation.

[0006] It has been known for LI/L2 receivers to use parallel RF pathsand/or RF switching of the input and/or local oscillator (LO) signals.For example, U.S. Pat. No. 5,883,597 discloses an LI/L2 GPS receiver inwhich the LO is switched between three frequencies to select “L1 only,”“L2 only” or “L1 and L2.” However, this requires the LO to be tunableover a very wide frequency range of about 696 MHz, from approximately1054 MHz to 1750 MHz, which makes on-chip integration difficult.Further, due to practical design limitations, this may require switchingbetween two or three tuned oscillators, which may result in excessivepower consumption for multiple voltage controlled oscillators (VCOs).Also, in the “L1 and L2” mode, this receiver may suffer a 3 dB noisepenalty due to image noise. Switching of the LO signal may also requireresynchronization of tracking loops, which reduces receiver responsetime for time sensitive applications.

[0007] U.S. Pat. No. 5,678,169, for example, discloses an L1/L2 receiverin which the VCO and LO frequency is fixed exactly halfway between theLI and L2 carriers, as in the “L1 and L2” mode of the above referred-toreceiver. This receiver uses switched L1 and L2 filters which eliminatethe problem of the 3 dB image noise. However, this receiver may not becapable of true simultaneous LI and L2 detection, since the L1/L2selection is done by RF switches before the mixer.

[0008] U.S. Pat. Nos. 5,040,240 and 5,736,961, for example, discloseL1/L2 receivers which use parallel RF paths for the downconversion. U.S.Pat. No. 5,040,240 uses a common VCO with a series of different dividersand multipliers for the L1 and L2 downconversions. However, due to theduplication of RF functions, these methods are not optimum for highintegration and low-power.

[0009] Therefore, those concerned with the development and use ofimproved dual frequency carrier signal receiver systems and methods haverecognized the need for improved systems and methods for enablingsimultaneous dual frequency capabilities without requiring radiofrequency switches or local oscillator switching.

[0010] Accordingly, the present invention fulfills these needs byproviding efficient and effective systems and methods for simultaneouslyreceiving or switching between dual frequency carrier signals in ahighly integrated, low power receiver.

SUMMARY OF THE INVENTION

[0011] Briefly, and in general terms, the present invention provides asystem and method for simultaneously receiving or switching between dualfrequency carrier signals.

[0012] By way of example, and not by way of limitation, the presentinvention provides a new and improved system for simultaneouslyreceiving or switching dual frequency carrier signals, without localoscillator switching or radio frequency switches.

[0013] More particularly, the present invention includes a sub-harmonicfrequency generator, which may include a sub-harmonic VCO, withdifferent harmonics of the sub-harmonic frequency VCO providing thelocal oscillator signals for the L1 and L2 carriers. Downconversion inthe sub-harmonic frequency generator or a first mixer then produces twointermediate frequencies (IF) for the L1 and L2 carriers. The VCOfrequency and harmonic orders may be chosen such that the differencebetween these two IF signals is twice the desired final IF. The final IFmay be obtained through a second mix in a second mixer with an LO signalthat is halfway between the L1 and L2 IF frequencies. Since these IFsignals generated in the first mixer are on either side of the LOfrequency they can be separated by having the second mixer be an imagereject mixer. The image reject mixer can be used to receive L1 and L2simultaneously using both its outputs, or to switch between L1 and L2.The selection is accomplished by interchanging the “I” and “Q” LO inputsignals of the second IR mixer. Since this switching is done at a lowerIF frequency it does not cause unlocking of the phase locked loop (PLL)or the receiver tracking loop.

[0014] This receiver architecture is chosen to minimize powerdissipation, while optimizing integration and performance. Operation ofan on-chip integrated VCO at a frequency three to four times lower thanthe L1/L2 RF carriers saves power in the VCO and PLL. Switching at theIF frequency consumes less power compared to RF or LO switching, anddoes not degrade the receiver noise figure. RF switches introducefront-end loss which degrades the receiver noise figure. Only oneexternal split band filter is required at the front end to reject thefirst image frequencies for the L1 and L2 downconversion. The secondimage is rejected by the image reject function of the second mixer.There is no 3 dB degradation for simultaneous L1/L2 herein.

[0015] A single fixed frequency VCO eliminates the need of LO switching,and eliminates the need of RF switches, while still providingsimultaneous L1 and L2 capability.

[0016] Although the preferred embodiment described is an L1/L2 GPSreceiver, the systems and methods described herein can be used for anydual frequency RF receiver.

[0017] The above and other objects and advantages of the invention willbecome apparent from the following more detailed description, when takenin conjunction with the accompanying drawing of an illustrativeembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The FIGURE is a circuit diagram of a dual frequency carriersignal receiver, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The present invention is directed to an improved system andmethod for simultaneously receiving or switching dual frequency carriersignals. The improved system and method provides efficient and effectivesimultaneous receiving or switching, without radio frequency switches orlocal oscillator switching. The preferred embodiments of the improvedsystem and method are illustrated and described herein by way of exampleonly and not by way of limitation.

[0020] Referring now to the FIGURE, which illustrates a system 10 forsimultaneously receiving or switching between dual frequency carriersignals, comprising a receiver 12 which is preferably a GPS receiver forthe L1 and L2 carriers. The front-end of the receiver 12 consists of adual band antenna 14 for receiving the dual frequency carrier signals,and a wide band low noise amplifier (LNA) 16, with 1.2GHz to 1.6 GHzbandwidth, for amplifying the L1 and L2 carriers. A split band surfaceacoustic wave (SAW) filter 18 is then used to pass the L1 and L2 bandsand reject other frequencies. The out-of-band rejection of this filter18 is adapted to be high enough to sufficiently attenuate the firstimage frequencies, as set forth below.

[0021] The system 10 further includes a sub-harmonic frequencygenerator, for generating a sub-harmonic frequency so as to enableharmonics of the sub-harmonic frequency to generate local oscillatorfrequency signals for the dual frequency carrier signals, and for mixingthe dual frequency carrier signals with the local oscillator frequencysignals, to generate distinct intermediate frequency signals for eachdual frequency carrier signal. The sub-harmonic frequency generator maycomprise a voltage controlled oscillator (VCO) 20, for generating thesub-harmonic frequency, and a first mixer 22, which may comprise asub-sampling mixer for mixing the dual frequency carrier signals withthe local oscillator frequency signals. The sub-harmonic frequencygenerator alternatively may comprise a sub-harmonic mixer, forgenerating the sub-harmonic frequency, and for mixing the dual frequencycarrier signals with the local oscillator frequency signals, the choiceof which as an alternative to the sub-sampling mixer may depend on thefrequency plan. The sub-sampling mixer 22 may be adapted to includeswitches comprising N-channel metal oxide semiconductor transistors. Thesignals from the first mixer 22 are input into a second mixer 24.

[0022] The VCO 20 comprises a sub-harmonic voltage controlledoscillator, adapted to generate a sub-harmonic frequency thereof and toenable harmonics of the sub-harmonic frequency to generate LO frequencysignals for the dual frequency carrier signals. The signal from the VCO20 is input into the first mixer 22, and is input into a divide by threedivider 26 and a divide by five divider 28, from which the I and Qphases are input into an IQ select switch 30. The IQ select switch 30selectively switches between the I and Q phases, and the I and Q phasesare input into the second mixer 24. The IQ select switch 30 enablesefficient selection and switching to be accomplished between the L1 andL2 signals by enabling the interchanging of the I and Q LO input signalsin the second mixer 24. The second mixer 24 is an image reject (IR)mixer, which is adapted to separately receive the L1 and L2 signals, andincludes a pair of outputs. It can be configured to simultaneouslyprovide both L1 and L2 signals using both mixer outputs and dual outputpaths, or to switch between the L1 and L2 mixer outputs using only oneoutput path. The selection in the second mixer 24 between the L1 and L2signals is preferably implemented by interchanging the “I” and “Q”signals of the LO frequency signals.

[0023] The IF signals generated in the first mixer 22 are preferably oneither side of an LO frequency signal, and are adapted to be separatedby the second mixer 24. The second mixer 24 is further adapted togenerate the final IF upon mixing with an LO frequency signal which isintermediate to the L1 and L2 IFs. The LO frequency adapted to be mixedwith the L1 and L2 IFs in the second mixer 24 is approximately halfwaybetween the L1 and L2 IFs. The final IF signal is input into a low-passfilter 32 and automatic gain control (AGC) amplifier 34 before beingsampled by an analog to digital (A/D) converter 36. The frequency andthe harmonics of the VCO 20 are preferably chosen such that thedifference between the first IFs is approximately twice the desiredfinal IF.

[0024] Several constraints influence the choice of a frequency plan forthe receiver 12 and the frequency of the VCO 20. For low powerdissipation in the VCO 20 and a PLL 38 it is desirable to have the VCOfrequency as small as possible. This may increase the sub-harmonicratio, which is the number of times the VCO signal must be multiplied inthe mixer 22 before mixing with the carrier. The preferred frequency ofthe VCO 20 is about 401.62 MHz. The noise figure of the mixer 22 mayincrease with increasing sub-harmonic ratios, which may degrade receiverperformance, and require more RF gain. A sub-harmonic ratio of 3 to 4 ispreferred, lowering the VCO power significantly while minimizing themixer noise. Another constraint is that the L1 and L2 IF signals afterthe first mixer 22 should be high enough to enable sufficient rejectionof the image frequencies by the RF SAW filter 18.

[0025] If n and m are the sub-harmonic ratios for L1 and L2respectively, and f_(vco) is the VCO frequency, then the first IFfrequencies for the L1 and L2 carriers are given by f_(IF1,L1)=|nf_(VCO)−1227.6 MHz , and by f_(IF1,L2) =|mf_(VCO)−1575.42 MHz |. The LOfrequency for the second mixer 24 is given byf_(L02)≅(f_(IF1,L1)+f_(IF1,L2))/2, and the final intermediatefrequencies are given by f_(IF2)≅|f_(IF1,L1)-f_(IF1,L2)|/2.

[0026] Preferably n=3 and m=4 for the sub-harmonic ratios. Usingconvenient integer dividers to generate the 2^(nd) LO and samplingfrequency, the optimum frequency plan is then given by:f_(VCO)=401.63MHZ, f_(IF1,L1)=31. 10 MHz, f_(IF1,L2)=22.71 MHZ,f_(L02)=f_(VCO)/15=26.78 MHz, f_(IF2,L1)=4.32 MHz, f_(IF2,L2)=4.07 MHzand sampling frequency f_(s)=fvco/24=16.73 MHz. Oversampling by a factorof approximately four eliminates any degradation due to noise folding,and provide samples that are close to 90 degree I, Q samples whichminimize processing loss. The first image frequencies for mixer 22 are62.2 MHz and 45.4 MHz away from the L1 and L2 carriers respectively.This does not put excessive demands on the split band RF SAW filter 18,which would require a 25 MHz to 30 MHz 3-dB bandwidth for each band.

[0027] The sub-harmonic mixer 22 should have sufficient conversion gainand sufficiently low noise figure for the n=3 and m=4 sub-harmonicmixing products, to minimize impact on the receiver sensitivity. Asub-sampling integrated switched capacitor implementation of the mixer22 is preferred for optimum performance at both the 3^(rd) and 4^(th)harmonics.

[0028] The first mixer 22 is preferably adapted to mix the thirdharmonic of the VCO 20 with the L2 carrier, and the fourth harmonic ofthe VCO 20 with the L1 carrier.

[0029] In accordance with the present invention, true simultaneous L1/L2reception capability is provided, with the flexibility to choosesimultaneous or switched operation, and without 3 dB degradation innoise figure.

[0030] In the present invention, an efficient means of switching betweenL1 and L2 at the IF frequency is provided by exchanging the I and Q LOsignals of the 2^(nd) IR mixer. Lower frequency switching at the IFdissipates less power than the RF and LO switching techniques of priorswitching, and also does not disturb the phase lock of the PLL andtracking loops. IF switching in the system after amplifier gain thereinalso eliminates the noise figure degradation caused by front-end RFswitches.

[0031] Pursuant to the invention, the sub-harmonic VCO for a dualfrequency GPS receiver operating at one fourth the LI frequency isadapted to save power dissipation in the VCO and PLL.

[0032] In accordance with the invention, on-chip RF functions andexternal components required for a high functionality GPS LI/L2 receiverare minimized. An external high-Q IF filter is not required, and the VCOcan be easily be integrated on the chip.

[0033] It will be apparent from the foregoing that, while particularforms of the invention have been illustrated and described, variousmodifications can be made without departing from the spirit and scope ofthe invention. Accordingly, it is not intended that the invention belimited, except as by the appended claims.

What is claimed:
 1. A system for simultaneously receiving or switchingbetween dual frequency carrier signals, comprising: a sub-harmonicfrequency generator, for generating a sub-harmonic frequency so as toenable harmonics of the sub-harmonic frequency to generate localoscillator frequency signals for the dual frequency carrier signals, andfor mixing the dual frequency carrier signals with the local oscillatorfrequency signals, to generate distinct intermediate frequency signalsfor each dual frequency carrier signal, wherein the local oscillatorfrequency signals are adapted to include I and Q phases; and an imagereject mixer, for separating the dual frequency carrier signals, and forswitching between the dual frequency carrier signals, responsive toexchanging the I and Q phases of the local oscillator frequency signals.2. The system of claim 1, comprising a global positioning systemreceiver, wherein the dual frequency carrier signals comprise L1 and L2GPS carrier signals, and the GPS receiver is adapted to simultaneouslyreceive or switch between the L1 and L2 GPS carrier signals.
 3. Thesystem of claim 1, wherein the sub-harmonic frequency generatorcomprises a voltage controlled oscillator, for generating thesub-harmonic frequency, and a mixer, for mixing the dual frequencycarrier signals with the local oscillator frequency signals.
 4. Thesystem of claim 1, wherein the sub-harmonic frequency generatorcomprises a sub-harmonic mixer, for generating the sub-harmonicfrequency, and for mixing the dual frequency carrier signals with thelocal oscillator frequency signals.
 5. The system of claim 1, furtheradapted to generate a final intermediate frequency, wherein the imagereject mixer is adapted to generate the final intermediate frequencyupon mixing with a local oscillator frequency signal which isintermediate the L1 and L2 intermediate frequencies.
 6. The system ofclaim 1, wherein the intermediate frequency signals generated in thesub-harmonic frequency generator are on either side of the localoscillator frequency signal in the image reject mixer, and are adaptedto be separated by the image reject mixer.
 7. The system of claim 1,wherein the image reject mixer includes a plurality of outlets adaptedto be connected therein, and is adapted to receive the dual frequencycarrier signals simultaneously through the plurality of outputs.
 8. Thesystem of claim 1, wherein the image reject mixer includes a pluralityof outlets adapted to be connected therein, and is adapted to switchbetween the dual frequency carrier signals through one of the pluralityof outputs.
 9. The system of claim 2, wherein the sub-harmonic frequencygenerator is adapted to mix the third harmonic with the L2 carrier, andthe fourth harmonic with the L1 carrier.
 10. The system of claim 2,wherein the sub-harmonic frequency is about 401.62 MHz.
 11. The systemof claim 2, further comprising a split-band surface acoustic wavefilter, for passing the L1 and L2 frequencies and rejecting otherfrequencies, adapted to be connected to the sub-harmonic frequencygenerator.
 12. The system of claim 2, further adapted to generate afinal intermediate frequency, wherein the frequencies of thesub-harmonic frequency generator signal and the sub-harmonic frequencygenerator harmonics are such that the difference between the distinctintermediate frequency signals is twice the final intermediatefrequency.
 13. The system of claim 4, wherein the sub-harmonic mixercomprises an integrated switched capacitor sub-sampling mixer.
 14. Thesystem of claim 12, wherein the local oscillator frequency which isadapted to be mixed with the L1 and L2 intermediate frequencies in theimage reject mixer is substantially halfway between the L1 and L2intermediate frequencies.
 15. The system of claim 13, wherein thesub-sampling mixer includes switches comprising N-channel metal oxidesemiconductor transistors.
 16. A method of simultaneously receiving orswitching between dual frequency carrier signals, in a system whichcomprises a sub-harmonic frequency generator, for generating asub-harmonic frequency so as to enable harmonics of the sub-harmonicfrequency to generate local oscillator frequency signals for the dualfrequency carrier signals, and for mixing the dual frequency carriersignals with the local oscillator frequency signals, to generatedistinct intermediate frequency signals for each dual frequency carriersignal, wherein the local oscillator frequency signals are adapted toinclude I and Q phases, and an image reject mixer, for separating thedual frequency carrier signals, and for switching between the dualfrequency carrier signals responsive to exchanging the I and Q phases ofthe local oscillator frequency signals, wherein the method comprises:generating a sub-harmonic frequency so as to enable harmonics of thesub-harmonic frequency to generate local oscillator frequency signalsfor the dual frequency carrier signals, and mixing the dual frequencycarrier signals with the local oscillator frequency signals, to generatedistinct intermediate frequency signals for each dual frequency carriersignal; and separating the dual frequency carrier signals, and switchingbetween the dual frequency carrier signals, responsive to exchanging Iand Q phases of the local oscillator frequency signals.
 17. The methodof claim 16, comprising a global positioning system receiver, whereinthe dual frequency carrier signals comprise L1 and L2 GPS carriersignals, and the GPS receiver is adapted to simultaneously receive orswitch between the L1 and L2 GPS carrier signals, wherein generating andmixing, and separating and switching comprises generating and mixing,and separating and switching the L1 and L2 GPS carrier signals.
 18. Themethod of claim 16, wherein the sub-harmonic frequency generatorcomprises a voltage controlled oscillator, for generating thesub-harmonic frequency, and a mixer, for mixing the dual frequencycarrier signals with the local oscillator frequency signals, and whereingenerating and mixing comprises generating in the voltage controlledoscillator, and mixing in the mixer.
 19. The method of claim 16, whereinthe sub-harmonic frequency generator comprises a sub-harmonic mixer, forgenerating the sub-harmonic frequency, and for mixing the dual frequencycarrier signals with the local oscillator frequency signals, and whereingenerating and mixing comprises generating and mixing in thesub-harmonic mixer.
 20. The method of claim 16, further adapted togenerate a final intermediate frequency, wherein the image reject mixeris adapted to generate the final intermediate frequency upon mixing witha local oscillator frequency signal which is intermediate the L1 and L2intermediate frequencies, further comprising generating a finalintermediate frequency in the image reject mixer.
 21. The method ofclaim 16, wherein the intermediate frequency signals generated in thesub-harmonic frequency generator are on either side of the localoscillator frequency signal in the image reject mixer, and are adaptedto be separated by the image reject mixer, wherein generating furthercomprises generating the intermediate frequency signals on either sideof the local oscillator frequency signal of the image reject mixer, andseparating the intermediate frequency signals in the image reject mixer.22. The method of claim 16, wherein the image reject mixer includes aplurality of outlets adapted to be connected therein, and is adapted toreceive the dual frequency carrier signals simultaneously through theplurality of outputs, further comprising receiving the dual frequencycarrier signals simultaneously through the plurality of outlets in theimage reject mixer.
 23. The method of claim 16, wherein the image rejectmixer includes a plurality of outlets adapted to be connected therein,and is adapted to switch between the dual frequency carrier signalsthrough one of the plurality of outputs, further comprising switchingbetween the dual frequency carrier signals through one of the pluralityof outputs in the image reject mixer.
 24. The method of claim 17,wherein the sub-harmonic frequency generator is adapted to mix the thirdharmonic with the L2 carrier, and the fourth harmonic with the L1carrier, further comprising mixing the third harmonic with the L2carrier, and the fourth harmonic with the L1 carrier, in thesub-harmonic frequency generator.
 25. The method of claim 17, whereinthe sub-harmonic frequency is about 401.62 MHz, wherein generatingcomprises generating a sub-harmonic frequency of about 401.62 MHz in thesub-harmonic frequency generator.
 26. The method of claim 17, furthercomprising a split-band surface acoustic wave filter, for passing the L1and L2 frequencies and rejecting other frequencies, adapted to beconnected to the sub-harmonic frequency generator, further comprisingpassing the L1 and L2 frequencies and rejecting other frequencies in thesplit-band surface acoustic waver filter.
 27. The method of claim 17,further adapted to generate a final intermediate frequency, wherein thefrequency of the sub-harmonic frequency generator signal and thesub-harmonic frequency generator harmonics are such that the differencebetween the distinct intermediate frequency signals is twice the finalintermediate frequency, further comprising generating a finalintermediate frequency wherein the difference between the distinctintermediate frequency signals is twice the final intermediatefrequency.
 28. The method of claim 19, wherein the sub-harmonic mixercomprises an integrated switched capacitor sub-sampling mixer, andwherein generating and mixing comprises generating and mixing in theintegrated switched capacitor sub-sampling mixer.
 29. The method ofclaim 27, wherein the local oscillator frequency which is adapted to bemixed with the L1 and L2 intermediate frequencies in the image rejectmixer is substantially halfway between the L1 and L2 intermediatefrequencies, and wherein generating a final intermediate frequencyfurther comprises generating a final intermediate frequency in which thelocal oscillator frequency is substantially halfway between the L1 andL2 intermediate frequencies.
 30. The method of claim 28, wherein thesub-sampling mixer includes switches comprising N-channel metal oxidesemiconductor transistors, and wherein mixing further comprises mixingin the N-channel metal oxide semiconductor transistor switches in thesub-sampling mixer.